Faster recovery from severe sepsis (or other disorders causing patchy/global hypoperfusion) should be accompanied by a faster disappearance of lactate from the bloodstream. The rate of lactate clearance was reported by Nguyen et al in Crit Care Med 2004 (among others) to predict survival from septic shock, and a strategy substituting lactate clearance of 10% for central venous oxygen saturation of 70% as a "goal" of goal-directed therapy was noninferior to standard care in a randomized trial in JAMA in 2010.(Lactate clearance is simply the change in lactate over a time period divided by the original lactate, reported as a percentage; e.g., lactate 4 --> 3 = 1/4 = 25% lactate clearance.)

Nichol et al mined their huge research database of previously collected data on >7,100 patients in 4 hospitals in Australia. There were 36,673 measurements of lactate in the first 24 hours of ICU admission. They only analyzed the 5,041 patients who had 2 or more lactate levels checked.

Results: The change in lactate over the first 24 hours was predictive of mortality, and superior to isolated measurements of lactate. When added to APACHE II scoring, they report achieving 90% accuracy in mortality predictions. Each increase in lactate of 1 mmol/L over 24 hours increased the risk of death by 15%. These relationships were also true over the first 6 hours after ICU admission.

They also performed a complex calculation of "time weighted average lactate" (using a multiplier for higher time spent at a particular value), which was even more predictive, but cumbersome to report and not usable in clinical practice.

They did calculate the percent change in lactate and reported it in their tables, but chose to go with the (I think) less informative absolute change in lactate for their findings. The percent change in lactate (i.e., lactate clearance described above) looks in their table to predict mortality about as well or slightly better than the absolute change.